Tell him to listen to the song properly, and sing along with it.
practice alot of practice, also what weve done is put the song on an mp3 and have the singer listen to it like 10 times in a row while singing along, also i guess you could play the track while the band plays at the same time and also you should record it once and let the singer see what hes doing wrong.

good luck to ya.
Some singers have timing like a mother! Others can't do it without alot of work. Its just practice practice practice.
Punch him in rhythm with the song. Everytime he gets it wrong punch the rhythm into him.

No in all seriousness just explain to him he's not in time, play him the song and show him the real rhythm and how he's getting it wrong and everything. Even record himself singing it and show him the difference.

Then again if he's not completely out of time and it doesnt sound bad then let him use his own rhythm. It'll make the song sound like more of a personalised reinterpretation by your band.
If someone else in the band knows the song stand next to him and sing it with him as you play it. That usually works for us. I'll sing it right in his ear as were jammin a song.
Let him know that he's off time and in which areas he goes off time. Recording him and having him listen is a great idea too. Besides that make sure he keeps practicing, both as a band and to the song.
WARNING:This post contains explicit portrayals of violence; sex; violent sex; sexual violence; clowns and violent scenes of violent excess, which are definitely not suitable for all audiences.
have him practice to a recording, a lot, singing is IMO the hardest thing to do in time. And unless he's seriously ahead or behind the beat then you could just let it slide, most bands don't even play their own songs in time all the time.
make Industrial and/or experimental electronic music? Join my group!

Quote by Heavens_To_Hell
Punch him in rhythm with the song. Everytime he gets it wrong punch the rhythm into him.

Ha ha ha.
That'd definately work.

I agree with grimms, get him to sing along with Pantera's recording of it and copy Phil Anselmo's phrasing and timing as exactly as he can, over and over again. Eventualy he'll get it nailed perfectly.
I actualy learned most of our set like this. I have a small CD player in my kitchen and whenever I'm doing the washing up or cooking a meal or whatever, I put on the CD of whatever I'm learning and sing along with it, over and over again. It bugs the life out of my wife and kids but it's appreciated by the rest of my band.